SI Units

The International System of Units (SI, or Système International d'Unités), is a set of measurement standards which defines (almost) all standards in terms of uniform natural phenomena, and form the base of the metric system.

Base SI Units

There are seven base SI units, with many derived units made from combinations of these. Base SI units are mutually independent. They consist of:

Quantity

Unit

Abbreviation

Definition

Time

Second

s

Duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of an atom of Cs-133

Length

Metre

m

Distance that light travels in a vacuum in 1/299,792,458th of a second

Current

Ampere

A

The constant current that would produce a force of 2x10-7 Newton between two conductors of infinite length and negligible cross section in a vacuum

Temperature

Kelvin

°K

1/273.16th of the triple point of water. The triple point is the temperature at which a substance exists in equilibrium in all three phases (solid, liquid, gas).

Amount

Mole

mol

The amount of substance which contains as many elementary entities as in 0.012kg of Carbon 12

Luminous Intensity

Candella

cd

Luminous intensity of a source which emits monochromatic radiation at 540 x 1012 Hz at radiant intensity of 1/683 watts per steradian